A known hybrid electric vehicle powertrain with dual power flow paths between an engine and vehicle traction wheels and between an electric motor and vehicle traction wheels will permit the vehicle to operate with maximum performance by managing power distribution from each power source. This includes managing the operating state of the engine, the electric motor, a generator and a battery.
The battery, the generator and the motor are electrically coupled. A vehicle system controller is interfaced with a transmission control module to ensure that power management for optimum performance and drivability is maintained.
The powertrain may comprise gearing that defines a parallel power flow configuration in which motor torque and engine torque are coordinated to meet a wheel torque command. The vehicle system controller may cause the engine to be shut down under certain operating conditions, such as during a steady-state highway cruising mode for the vehicle, so that the vehicle may be powered solely by the electric motor. At this time, the battery acts as a power source for the motor. If the battery state-of-charge becomes reduced below a calibrated threshold value during the all-electric drive mode, the engine may be started to charge the battery and to provide a mechanical power source to complement the electric motor torque.
An example of a hybrid electric vehicle powertrain of this type may include a planetary gear set that is used to direct engine power to either an electric power flow path or a mechanical power flow path. Such a powertrain is disclosed, for example, in U.S. Pat. No. 7,268,442, which is assigned to the assignee of this invention. That powertrain includes a planetary gear set wherein the sun gear of the planetary gear set is drivably connected to the generator, the engine is drivably connected to the carrier of the planetary gear set and the motor is drivably connected to the ring gear of the planetary gear set. The power flow path is split by the planetary gear set when both the engine and the motor are active.
If the hybrid electric vehicle powertrain is a so-called “plug-in” powertrain, the motor will be operated for a significant period of a total driving event while the engine is off. A battery charge depletion strategy then is used to supply electrical energy to the motor until a battery state-of-charge depletion threshold is reached. The battery, following charge depletion, then may be charged by a public utility electric power grid in preparation for a subsequent driving event.
When the engine speed equals zero during all-electric drive, the generator will move at a speed that is a multiple of the motor speed, depending upon the overall gear ratio of the planetary gear set. This may create a problem related to durability of bearings for the gearing and the generator. This feature limits the road speed to a value that is less than optimum. This also may reduce available torque needed to start the engine when the battery state-of-charge falls below a predetermined threshold during a given driving event before an opportunity exists for recharging the battery using the utility power grid. A need thus exists for a powertrain architecture that would be designed to avoid over-speeding of the generator during operation in an all-electric drive mode.